Voltage-controlled oscillators (VCOs) are commonly used in communication systems where accurate control of frequency or phase of a given signal is desirable. Various types of oscillating circuits such as relaxation oscillators, ring oscillators and inductor-capacitor (LC) oscillators have been used to implement VCOs.
The VCO performance may be affected by its tuning mechanism. VCO tuning based on interpolating two different RC delay paths has shown better performance compared with conventional current starving type ring oscillator.
For example, in a prior art RC delay interpolating ring oscillator, such as that described in U.S. Pat. No. 4,884,041 by Walker, two paths with different number of delay cells are used in combination with a linear combining circuit to provide a ring oscillator capable of varying oscillation frequency within a certain specified range. However, due to the lack of high-Q elements in the delay cells, this particular ring oscillator, does not provide good noise performance. In addition, due to the process limitation of the delay cells, the operating frequency of the oscillation signal is limited.
For very high frequency applications, LC VCOs have been favored because of their improved frequency stability and superior phase noise performance. The most common LC VCO is tuned by a diode varactor. Varactors, however, suffer from a number of shortcomings including, for example, process dependent, bias dependent and nonlinearity problems.
More importantly, the operating frequency of an LC VCO is controlled by the resonant frequency of the LC tank, which is given by f=1/(2xcfx80LC). According to this equation, in order to increase the resonant frequency, either inductance L and/or capacitance C need to be reduced. When the inductance L is reduced, the impedance of the LC tank will accordingly decrease. As a result, more power is required for the VCO to maintain oscillation. The need for additional power, however, leads to use of larger transistors which introduce more parasitic capacitance. The increase in parasitic capacitance contributes to the total capacitance and thus decreases the resonant frequency.
Furthermore, varactors constitute a major portion of the total capacitance. The use of varactors is contrary to the goal of reducing the total capacitance in order to increase resonant frequency. Moreover, the tuning range and driving capability requirements of varactors often limit the reduction of the total capacitance. It is, therefore, desirable to provide an LC VCO which is implemented without using varactors.
The present invention provides methods and circuitry for implementing inductor-capacitor phase interpolated voltage-controlled oscillators using standard CMOS process technology.
In an exemplary embodiment, the present invention includes two phase shifters and an interpolator. The two phase shifters are coupled to the interpolator in a loop configuration. The differential output from the two phase shifters are fed into the interpolator. The differential output of the interpolator, in turn, is fed back to the two phase shifters.
The two phase shifters and the interpolator respectively utilize LC tanks as tuned circuits. The respective resonant frequencies of the LC tanks of the two phase shifters are different but close to each other. The resonant frequency of the interpolator is at the center of the VCO tuning range. The resonant frequency of one phase shifter is higher than that of the interpolator; while the resonant frequency of the other phase shifter is lower. The interpolator control is operated in the linear region and the signal gain from the two phase shifters may be continuously tuned when signals from the two phase shifters are combined at the interpolator. Both VCO and VCO control operate in a fully differential manner.
In an exemplary embodiment, the present invention provides a voltage-controlled oscillator including: a first phase shifter and a second phase shifter each having an inductor-capacitor resonator circuit, wherein the respective resonant frequencies of the first and second phase shifters are different, and an interpolator having an inductor-capacitor resonator circuit, wherein the resonant frequency of the resonator circuit of the interpolator is set between the respective resonant frequencies of the first and second phase shifters.
The two phase shifters utilize CMOS differential-pair as a tuned buffer, which has a pair of high frequency input and a pair of high-frequency output. The interpolator utilizes a multiplexor configuration, which has two pairs of high-frequency input and a pair of high-frequency output, as well as a pair of low-frequency control signals. All the high frequency signals and low-frequency control signals operate in a fully differential manner. The output from the two phase shifters are fed into the high frequency input of the interpolator. The output of the interpolator, in turn, is fed back to the inputs of two phase shifters. Signals from the first and second phase shifters are selectively combined by the interpolator according to the control signal. Thus, the VCO can be continuously tuned near the resonant frequency of the interpolator.
Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.